Automatic gain control circuits



Se t. 10, 1940. Q 5 BULL 2,214,608

AUTOMATIC GAIN CONTROL CIRCUITS.

Filed Oct. 30, 1936 Anode (Bug-rent Grid. Potential;

= INVENTOR CABOT s. BULL ATTORNEY Patented Sept. 10, 1940 Cabot Seaton Bull, Uxbridge, Middlesex, England, assignor to Electric & Musical Industries Limited, Middlesex, England Application October 30, 1936, Serial No. 108,461

In Great Britain November 6, 1935 9 Claims.

This invention relates to thermionic valve circuits in which provision is made whereby the gain of the circuitcan be controlled.

It has been proposedto provide an amplifier having gain control means in which signals are applied in push-pull to the control grids of two valves the output being taken from the anodes of the valves in parallel and in which gain control is effected by'varying the bias on the control grids simultaneously and in opposite senses; A gain control circuit of this kind is not altogether satisfactory owing, amongst other difliculties, tothe necessity of using the two valves.

In certain types of valves which have essentially between the cathode and anode in the order named an inner control electrode or grid operating at arelatively low negative potential, a

screening grid maintained at a high-positive potential and an outer control electrode 01 grid also operating at a relatively low negative potential, it is known that the slope of the characteristic curve, that is to say anode current plotted against the voltage of the outer control grid, is capable of being varied in dependence on the voltage of the inner control grid. A valve of this kind will, for the sake of convenience, hereinafter be referred to as a valve of the type described. 1

In practice valves of the type described may contain further electrodes; in particular an additional screening grid may be provided situated between the outer control grid and the anode. Such a valve is commonly known as a hexode, and is suitable-for use in the present invention;

It has been found that with valves of the type described, iffthe same signals are applied to both control grids, the operating characteristics may be made to differ greatly fromthose usually obtained, according to the relative phase of thesignals applied to the two grids and the way in which their biases are varied and it is the object of the present invention to arrange the phase of the input signals and the bias applied to the grids in such a way that a characteristic eminently suitable for gain control purposes is obtained. According to the invention, gain control is effected by applying signals in opposite phase to the control electrodes of a valve of the type described, applying a steady biasing potential to one of the control eelctrodes, maintaining the other control electrode at a substantially difierent steady biasing potential and varying simultaneously the biasing potentials of said control electrodes in opposite senses.

The circuit for carrying out such method comprises avalve of, the type described, means for applying signals to be amplifiedin opposite phase to the two control electrodes of said valve. an output circuit associated with the anode of the valve from which amplified signals are derived, means for applying a steady biasing potential of a prea characteristic of parabolic form is eminently suitable for distortionless volume control.

The means for varying the potentials of the control electrodes may be operated manually or may operate automatically in dependence upon the amplitude of the applied signal. Preferably, the grid bases of the two control electrodes are equal but in cases where the grid bases are not equal, it is preferred to arrange that the changes of potential of the control grids are proportional to the lengths of their grid bases.

In order that the said invention may be clearly understood and readily carried into effect, the same will now be' more fully described with reference tothe drawing accompanying my specification in which:

Fig. 1 shows a circuit according to the invention,

Fig. 2 is an explanatory diagram, and

Fig. 3 shows a modification of Fig. 1.

Referring to Fig. 1, a hexode valve l which is one form of valve of the type described has a cathode 2, an anode 3, an inner control grid 4, a screen grid 5, an outer control grid 6 and a second screen grid 1; The cathode is supplied with a heater 8 for connection to a suitable source of heating current. Signals to be amplified are fed to terminals 9 of the primary winding 10 of high frequency transformer II. The secondary winding l2 of transformer II has its ends coupled through coupling condensers l3, [4 to controls grids 4 and 6 for applying signals in opposite phase thereto, and its center tap is connected to cathode 2 of valve I. The anode 3 is coupled through a tuned circuit comprising an inductance l5 and a condenser [6 in parallel to a terminal I! to which a suitable source (not shown) of anode current is connected, the source having its negative terminal earthed. An output voltage istapped off between terminal [8 (which is connected to anode 3 and earth).

The cathode 2 of valve I is earthed. A terminal I9 is connected through a resistance 20, shunted by a decoupling condenser 2|, to earth and also has connected to it the negative terminal of a source of potential (not shown) the positive terminal of which is earthed. Coupled to inductance l5 are two coils 22 and 23. Coil 22 is connected in series with a diode 24 and a load resistance 25, the latter being shunted by a condenser 25. One end of resistance 25 is earthed and the other end thereof is connected through a high resistance 21 to inner control grid 4. Coil 23 is connected in series with a diode 28 and a load resistance 29, the latter being shunted by a condenser 30. One end of resistance 29 is connected to terminal I9 and the other end is connected through a high resistance 3| to outer control grid 6. The screen grids 5 and l are connected together and maintained at a suitable positive potential with respect to the cathode.

The characteristic of valve l is shown graphically in Fig. 2 in which negative values of inner control grid potential are plotted as abscissae and anode currents as ordinates. The point B indicates the point at which the potential of the inner grid is sufficiently negative to reduce the anode current to Zero. This point is in the ideal case (which is closely approximated in practice) independent of the potential of the outer control grid. It will be assumed that point 38 corresponds to a negative potential (with respect to the oathode) of 4 volts on the inner control grid. The curves BC, BD, BE, BF and B (which, for con venience, are shown straight but in practice will be found in most cases to vary from the linear form shown), show respectively the relation between anode current and inner control 'gridpotential for potentials on the other control grid of 0, 1, 2 3, and 4 volts negative with respect to the cathode. In this particular case (which is a preferred arrangement) it has been arranged that the grid-bases of the two control grids are equal. That is, the potential on one control grid necessary to reduce the anode current substantially to zero is equal to the potential on the other control grid necessary to reduce the anode current substantially to zero. If it is arranged that the sum of the bias potentials on the two control grids is 4 volts, then the relation between anocle current and inner control grid potential is the curve OAB.

In the circuit of Fig. 1, it is arranged that the source connected to terminal l9 has a potential equal to that required to bias the outer grid 6 to cut-off when no signal is being applied to terminals 9. Under these circumstances the inner control grid is at cathode potential and the operating point is therefore the point 0 of. Fig. 2. When a modulated carrier is applied to terminals 9, grids i and 6 are swung equal amounts in opposite directions and therefore the instantaneous sum of the potentials on these grids remains constant. The operating characteristic is therefore curve OAB. At the point 0 the slope of this curve has its greatest value within the operating range and the gain of the amplifier is therefore at its highest value.

A part of the signal is rectified in diodes 2d, 28 and the rectified current sets up a potential difference across load resistances 25, 29. .It is arranged that these potential difierences are equal in magnitude but that grid 4 becomes more negative-and grid 6 less negative with increase in amplitude of the applied signal. The operating point thus moves from 0 towards A along the curve and the gain of the amplifier diminishes due to the decrease of slope of the characteristic at the operating point. At A the slope is zero and it is therefore possible to obtain a very wide variation of gain for a comparatively small con-.

trol potential. Furthermore, the curve OAB is substantially parabolic and hence the third differential of anode current with respect to inner control grid potential is substantially zero and, therefore, cross-modulation is substantially absent.

By reversing the connections from resistances 21 and 3| to grids 4 and E, thebias values on the grids are interchanged and the maximum condition (no applied signal) is indicated by the point B instead of the point 0. This arrangement has the advantage that the cathode emission current is limited by the inner grid in the maximum gain condition, and amplifier noise due to electrons repeatedly passing through grid is, therefore, minimized.

If the grid-bases of the two control grids are not of equal length, it is preferably arranged that the ratio of the change of bias potential of one grid to the corresponding change of bias potential of the other grid is equal to the ratio of the length of the grid'base of the one grid to the length of the grid base of the other grid. The bias of the grids may be caused to satisfy this condition by connecting the grid with the shorter grid base to a tapping point on the load resistance (25 or 29) across which the variable bias voltage is set up. In this way the change of bias on one grid is made less than the change of bias on the other grid. It is also preferred in the case of unequal grid bases to ensure that the signal amplitude applied to the two grids is proportional to the ratio of their grid bases. This may be effected by connecting the cathode of valve I to a point other than the center tap on coil I2.

Instead of controlling the gain automatically in dependence on the applied signal strength as in the arrangement of Fig. 1, the gain may be controlled manually, for example, by two potentiometers ganged together to vary the potentials of the two control grids in opposite senses.

In the modification of Fig. 1 which is shown in Fig. 3, like parts bear like reference numerals. In Fig. 3, coil 23, the diode28, resistances 20 and 29 and condenser 3!] of Fig. 1 are omitted and in their place there is included a bias resistance 32 shunted by a decoupling condenser 33 connected between the cathode 2 and earth. The end remote from outer control grid 6 of resistance Si is earthed.

The arrangement is such that, in the absence of an applied signal, the potential difference across resistance 32 is equal to the cut-01f voltage of the outer control grid. When the outer grid is cut on, no anode current flows, but, since the inner control grid is at cathode potential, the cathode emission has its greatest value (Within the operating range) this emission all flowing to the screen grid 5. When a signal is received, rectification takes places in diode 24 and a negative bias is impressed on inner control grid 4. This causes the total cathode emission current to diminish and, therefore, the negative bias on outer control grid 6' is reduced. This allows anodecurrent to flow and, by suitable constuction of. the valve and choice of components, it can be arranged that the changes of potential of the control grids are proportional to the lengths of their grid bases. As in the previously described arrangement the valve is preferably so constructed that the grid bases are of equal length.

It is to be understood that what is meant, by the term grid base is the range on the characteristic measured in terms of bias voltages over which the making bias of the valve may be varied without introducing an appreciable amount of distortion due to rectification or flow of grid current.

It is also to be understood that while in the preferred embodiments of the invention, it is desirable to apply the signal to the control grids in proportion to the lengths of the grid bases and to vary the biasing potentials in proportion to the grid bases, satisfactory gain control can however be effected in cases where this proportionality is not strictly adhered to.

Having now particularly described and ascertained the nature of my said invention and in what manner the same is to be performed, I declare that what I claim is:

1. In a signal wave transmission network of the type including an electron discharge tube provided with a cathode, an output electrode and at least two control electrodes, means applying signal waves to said control electrodes in opposite phase, means establishing one of the control electrodes at a steady negative potential of predetermined value, means establishing the other control electrode at a steady potential of different value, and means simultaneously varying the potentials of said control electrodes with respect to said values in opposite polarity senses said last means comprising a signal rectifier connected to one control electrode, and an impedance in the space current path of said tube connected to the second control electrode.

2. An amplifying circuit comprising a valve of the type comprising a cathode, anode and at least two control electrodes therebetween, means for applying signals to be amplified in opposite phase to the two control electrodes of said valve, an output circuit associated with the anode of the valve from which amplified signals are derived, means for applying a steady biasing potential of a predetermined value to one of the control electrodes, means for establishing the other control electrode at a substantially different potential value, and signal-controlled means for automatically varying the potential of said control electrodes simultaneously and in opposite senses for the purpose of varying the gain of said circuit.

3. A control circuit as in claim 2, wherein said '7. automatic varying means includes said output circuit coupled to a further circuit which rectifies a part of said output circuit signals and applies said rectified output to the control electrodes for automatically varying the potential of said control electrodes in opposite senses in dependence upon the average amplitude of the applied signals.

4. A circuit as in claim 2, wherein the automatic varying means comprises a circuit coupled to said output circuit and includes two diode rectifiers, the outputs of which are equal in magnitude and opposite in sense.

5. In a signal wave transmission network of the type including an electron discharge tube provided with a cathode, an output electrode and at least two control electrodes, means applying signal waves to said control electrodes in opposite phase, means establishingone of the control electrodes at a steady negative potential of predetermined value, means establishing the other control electrode at a steady potential of different value, and signal-actuated means for automatically and simultaneously varying the potentials of said control electrodes with respect to said values in opposite polarity senses thereby tocontrol the gain of said tube.

6. In a signal transmission circuit, a tube of the type comprising a cathode, an anode and at least two cold electrodes therebetween, means for applying signals to said two cold electrodes in different phase, an output circuit coupled tosaid anode, means for establishing one of said cold electrodes at cathode potential, means establishing the other cold electrode at a negative potential with respect to said one cold electrode potentiaLvand signal-controlled means for varying the potentials of said cold electrodes in opposite polarity sense to change the tube gain.

7. In a signal transmission circuit, a tube of the type comprising a cathode, an anode and at,

least two cold electrodes therebetween, means for applying signals to said two cold electrodes in difierent phase, an output circuit coupled to said anode, means for establishing one oi'said cold electrodes at cathode potential, means establishing the other cold electrode at a negative potential with respect to said one. cold electrode potential, means for varying the potentials of said cold electrodes in opposite polarity sense to change the tube gain, said tube inclduing a positive screen electrode between said two cold electrodes, and said varying means being responsive to the amplitude of said signals. v

8. In a signal transmission circuit, a tube of the type comprising a cathode, an anode and at least two cold electrodes therebetween, means for applying signals to said two cold electrodes in difierent phase, an output circuit coupled to said anode, means for establishing one of said cold electrodes at cathode potential, means establishing the other cold electrode at a negative potential with respect to said one cold electrode potential, and means for varying the potentials of said cold electrodes in opposite polarity sense to change the tube gain, said varying means ineluding signal rectifiers for producing potentials I of. opposite polarity and connections for applying the potentials to said control electrodes.

9. In a signal transmission circuit, a tube of the type comprising a cathode, an anode and at least two cold electrodes therebetween, means for applying signals to said two cold electrodes in,

potential onsaid impedance, means for rectifying signals to provide a negative potential, means for applying the last potential to said one cold electrode thereby to decrease the space current flow through said impedance.

CABOT SEA'ION BULL. 

